"Long before contemporary urbanist catchphrases like “gentle density” and “missing middle” housing were invoked to promote modestly scaled multifamily construction, triple-deckers embodied the idea. The original threeplex. Packed tightly onto narrow lots, sometimes in semi-detached pairs known as “perfect sixes,” the boxy structures — close to workplaces or linked to public transit— offered an attainable alternative to more expensive single-family homes, all in service to a healthy local economy. This was state-of-the-art workforce housing, New England-style." 

https://www.bloomberg.com/news/features/2023-05-17/three-cheers-for-the-triple-decker-boston-s-iconic-cheap-housing?utm_medium=email&utm_source=newsletter&utm_term=230517&utm_campaign=QH14LDDWX2Q601